Conventional molding methods, particularly methods for injection molding of thermoplastics, generally involve the cycling of a single, usually expensive, hard, permanent mold very rapidly in conjunction with expensive molding equipment. Molds are usually made of steel or other metal alloys and are designed and economical only for large production runs. The high cost of mold production, as well as the high molding pressures used, impose size constraints on the mold and hence the size of the part which can be produced.
Thermosetting liquid resin injection or transfer molding methods generally use lower pressures with often less expensive equipment but tooling costs in processes using matched-die molds are also generally high. Lower pressures enable the use of larger molds and hence larger parts, within practical limits, may be produced. Lower-cost tooling made of soft metals such as aluminum as well as polymers or composites are also being produced but are usually used for testing purposes, proto-typing or in short production run applications. Such tooling, however, is used and well suited to the production of large parts using liquid resins, often with reinforcements, in a variety of molding processes.
In all cases where hard molds are used, mold geometry, and therefore, part geometry, are strictly constrained to ensure mold separation and part de-molding. Both require part draft and the latter often requires an ejector-pin system built into the mold. Heat is used in many processes (all thermoplastic molding and as a curing agent or accelerator in many thermoset processes) and thus shrinkage factors, part thickness, residual stresses etc., greatly influence mold design. The cure or solidification time of the material molded is critical to the efficiency of the process and places further constraints on both mold design and material selection. Steel or metal-alloy mold production is a time-consuming process and there is a considerable lag between mold design and part manufacture. While tooling and equipment costs are high, the molding processes in which they are used are highly efficient and labor extensive.
Plastics casting methods, on the other hand, are generally labor intensive with low rates of production, but tooling and equipment costs are low. Low-viscosity thermosetting plastics are mostly used as many thermoplastics have poor flow characteristics for this application though some hot-melt casting is carried out. Many molds are permanent or semi-permanent and can be made of a variety of materials and be of large size. Molds used in simple casting are open or vented to the atmosphere and filled by directly pouring the material, usually liquid resin, into the mold, resulting in parts with little or no internal stresses but dimensional accuracy is variable and voids are common. Voids may be eliminated by placing the filled mold in a pressure vessel and allowing the resin to cure in the pressurized atmosphere. Small and large hollow items are often produced by various specialized casting processes such as static, slush and rotational casting.
In many metal-casting processes, the molds are single-use and consumed in the process, the mold being destroyed to release the object molded. In these processes, the geometry of the part molded may be very complex as there are no mold separation constraints on part design. The molds used must be continuously produced and are usually made of inexpensive or re-usable materials. Molds used for casting, like all molds, define the negative shape of the object to be molded and are produced from solid positive master patterns or from double half-positives of the object and any other elements required in the mold. Master patterns are generally inexpensive to produce and may be made from a variety of materials. These patterns are often copied in a more suitable material to provide multiples of the tooling required to make the production molds. This tooling is generally inexpensive and may be produced rapidly, requiring far less up-front capital investment and time compared with tooling for molding operations but requires an ongoing mold production operation, essentially transferring much of the tooling costs to the production side of the manufacturing operation.
Some plastics casting processes use one- or two-part elastomer molds to mold a variety of thermosetting resins. These molds, being flexible, can handle complex shapes with undercuts etc., and produce multiple castings but the mold surface degrades with use and the mold must be replaced after a certain number of castings. The one-part molds are usually produced by pouring the activated elastomer over a pattern enclosed in a box-frame to produce a self-supporting mass of elastomer. Alternatively, the pattern may be covered with a thin layer of the elastomer which, when cured, is itself covered by a hard material to produce an open skin-mold supported against gravity by the conforming backing material (known as a mother-mold) to comprise a compound mold. The compound-type mold is the most economical as a minimum of elastomer is consumed in the process. Both types are laid flat in use and the object produced has an unfinished section.
Two-part molds are produced by embedding master patterns in clay to the parting line, enclosing in a box-frame and covering with elastomer. When cured, the box is inverted, the clay removed and the second mold half poured against the first. These mold types are usually flat-backed, self-supporting masses of elastomer clamped together between rigid plates and stood vertically. The molds have opening(s) on the top edge for filling and venting and produce a completely finished part.
All conventional fully closed molding processes involve the use of hard molds made of metal or other materials and such molds must be designed to allow their separation and part removal. A minimum draft angle in the part is usually required and undercuts cannot be tolerated unless molding flexible parts. The majority of molding processes in current use involve thermoplastics as the molding material due to ease of molding and material recyclability. Molding of thermosetting plastics is often difficult due to their adhesive qualities and the use of release agents is often required. In general, for most closed molding processes, molds contain one or more cavities with runners to conduct the molding material from the injection port to each cavity. The runners narrow and connect to each cavity through one or more gates which have as small a diameter as possible, depending on the viscosity of the material being molded and the size of the cavity. The runners and cavities are filled with a fluid molding material which solidifies to produce the part(s) attached to the runner system. The mold is then opened and the molding removed by hand or ejected from the mold by ejector pins. The part(s) may be cut away from the runner system at the gates while being ejected or afterwards or may be shipped still attached and cut away by the user. Both the ejector pins and the gates generally leave scars on the part, which are often undesirable. Also the high cost of metal molds often limits the size and number of cavities to a practical minimum with the required number of parts being produced by cycling the mold the required number of times.
The primary objective of this invention is to provide a new mold\tooling system, for use in various conventional and novel plastics and composites molding applications, that overcomes disadvantages of the prior art molding systems described above. This objective is realized in a compound tooling system, based on polymers as the primary tooling material, which divides the mold into separate parts, based on the different functions which the mold, as a whole, has to carry out in the course of any single molding cycle.
Every molding cycle consists of three basic steps, mold filling, material solidification and part removal, and therefore all molds are required to define the shape, facilitate filling and solidification and allow de-molding of the part. Predictably, molds typically comprise different sections, or zones, that focus on one or another of these functions, and it is a principal objective of the invention to separate the mold into parts in parallel with these functions. The shape-defining cavity geometry of any mold is unique while the necessity for support of that shape is common to every mold. However many molds in current use are supported by a simple flat backing plate and the mold body being supported both maintains the shape-defining cavity and also contains systems dedicated to filling, solidification and part removal. Metal molds incorporating such systems are expensive and size-limited but have the benefit of permanence; on the other hand, incorporating such features in an impermanent polymer mold would be uneconomical. It is therefore a principal objective of the present invention to provide a practical and economical polymer-based alternative to metal tooling for many applications, and to do so with few limits on part size or length of production run.
This objective is realized in a system whereby the shape-defining surface of the mold is a separate, replaceable and inexpensive part used in conjunction with the permanent mold body and backing-plate which, being permanent, make it economically feasible to add features or systems to aid in the molding cycle. The tooling of the invention combines features used in molding and in casting practices, incorporating certain advantages of each, while limiting certain drawbacks of both. Tooling production is designed to be rapid and mold size is limited only by practicality. Reinforcing materials, inserts etc., may be molded and the system used to produce composite components. The system may be used to produce reinforcement pre-forms which may be subsequently over-molded or co-molded with others to produce more complex composite parts. The invention has, therefore, the added objective of providing a tooling system which allows a Variety of manufacturing processes great versatility with regard to the shape, size and quality of the component which may be produced.
Compound mold design using mold units according to the invention is identical in many ways to conventional mold design since the same objective is sought in generally the same way. While the size of any conventional mold is chosen according to the size of the part or number of parts, the comparable compound mold will be based on the size of the appropriate standard backing-plate (where a standard system is used). The low cost of the compound mold of the invention, on the other hand, allows the production of a larger mold or of one with a higher number of cavities producing more parts per cycle. Heating\cooling has long been a problem with solid metal molds due to the impracticality of providing conformal systems and such systems may be uneconomical to install in polymer molds of limited life-span. In the compound mold of the invention these systems are easily and inexpensively installed in the backing mold which has an unlimited life-span.
While the master tooling may be produced by hand or by conventional or computer-aided machining, the invention has the further objective of providing tooling which can be produced directly by computer-aided design through various rapid proto-typing technologies. Rather than use the rapid proto-typing system for proto-typing purposes only, the part file may be modified to directly produce the master molds which may be finished and copied depending on production requirements. The design file may then be modified to produce the support tooling required for the system. The invention essentially seeks to greatly increase the utility and application of the various rapid proto-typing technologies available and greatly reduce the time required for tooling production. By reducing initial tooling costs and production time, and by transferring much of the tooling costs to production rather than capital, the invention seeks to make shorter production runs more economical especially as lower cost production processes may often be used.
A precursor of the present invention was a tooling system, here called the “prototype tooling systems”, consisting of multi-cavity elastomer molds supported by rigid backing-plates, which established the advantage of using elastomers in the ease with which molds can be produced, and the ease with which these molds can be used to mold liquid resins. This “proto-type system” and its associated transfer molding process were developed to produce small, complex and detailed plastics parts and proved demonstrably capable of molding the types of parts for which they were designed, within certain size and geometrical limits. Beyond these limits, certain drawbacks of both a practical and economic nature become apparent, including dimension-loss across the parting plane due to mold compression, distortion of the cavities due to the inherent instability of the elastomer, and lastly, the comparatively large amount of elastomer used to produce a stable mold versus the amount actually consumed by the molding process. The compound tooling system of the present invention was therefore developed in part also to address and to overcome these drawbacks of the proto-type system.
The compound tooling system comprising this invention was designed to overcome these drawbacks and the key feature of the system is the vacuum-bonding of an elastomer skin-mold to the backing-mold. Since only a shallow surface layer of the elastomer mold is eroded during the molding process, the elastomer mold needs only to be slightly thicker than this erosion zone over the mold surface. The rigid backing-mold conforms to the skin-mold shape and supports it while the vacuum bond rigidizes it. The amount of elastomer used to make a mold is thus minimized and distortion of the cavities is eliminated. The elastomer skin-molds are easily moldable using hard polymer master molds. Dimension-loss due to compression is eliminated by modification of these master production molds so as to add elastomer to the mold mating surfaces sufficient to take up the compression. The clamping system holds the mold closed and sealed under adequate compression with no dimension loss while the backing-mold vacuum holds the cavities to design specification. The skin-mold and backing-mold are, in turn, supported by a flat, rigid backing-plate carrying standard features such as the clamping system and lifting hardware etc. and, being a necessity common to all compound molds, is designed to be one of a series of standard sizes. The custom skin and backing-mold may then be designed to suit one of a range of standard sized backing-plates, sufficiently comprehensive to accommodate any molding requirements.